Liquefied Petroleum Gas (LPG), as one of the important petroleum products produced by oil catalytic cracking apparatuses, contains a large quantity of propylene and C4-olefins, and can be separated and purified for use as a starting material for producing polyolefins as well as a starting material for etherification and alkylation processes for producing a blending component of a high-octane clean gasoline. However, LPG produced by catalytic cracking apparatuses contains an amount of sulfides as impurities, which may affect its subsequent processing and utilization. In particular, mercaptans contained in LPG (mainly methyl mercaptan and trace ethyl mercaptan) are toxic, malodorous and weakly acidic, and tend to cause corrosion of facilities and catalyst poisoning in the post-processing units. Accordingly, LPG sweetening has been necessary.
The first widely-used LPG-sweetening technique is sweetening by catalytic oxidation, developed by Universal Oil Products (UOP) Company. The company disclosed the method in U.S. Pat. No. 2,882,224 in as early as 1958, now known as the Merox liquid-liquid extraction and oxidation sweetening process. The sulfonated cobalt phthalocyanine or poly(cobalt phthalocyanine) catalyst used specially for this process can promote oxidation of the thiolate ions, as extracted with an alkaline solution, by the oxygen in air to produce a disulfide, which can subsequently be separated so that the purpose of removing mercaptans from LPG can be achieved. The sulfonated cobalt phthalocyanine catalyst in the process is used while dissolved in the alkaline solution, and thus easy to be deactivated due to aggregation. Therefore, it is necessary to continuously inject fresh alkaline solutions, which results in a short cycle period of the alkaline solution and large emission of waste alkaline solutions, bringing tremendous pressure on environmental protection.
As regards the sweetening by catalytic oxidation, UOP has been committing itself to improving the stability of the catalyst, and reducing or avoiding use of caustic alkali to prevent environmental pollution. The technical solutions disclosed in U.S. Pat. Nos. 2,988,500, 3,108,081 and 4,049,572 were a series of improvements made by UOP to the catalytic oxidation sweetening process and the catalyst. However, a small amount of alkaline solution or alkaline aids are still necessarily used in the sweetening process of the methods disclosed in these patents, which is still disadvantageous to environmental protection.
CN1706549A relates to a double-effect catalyst and its preparation method, wherein the catalyst is a solid formed article having a manganese compound as an active component and showing catalytic activity in both t-butyl hydroperoxide decomposition and mercaptan conversion. The double-effect catalyst can reduce the mercaptan sulfur in LPG to 0.1 μg/g or less. However, since t-butyl hydroperoxide serves as an oxygenating agent, it not only provides reactive oxygen species upon decomposition, but also produces isobutanol and leaves it in the LPG product, thereby increasing the cost for subsequent separation and purification.
Due to the above defects present in the catalytic oxidation method, an etherification sweetening method was developed later on. The etherification sweetening technique has been used in the sweetening process for a catalytic cracking gasoline in a plant by virtue of its advantages such as a mild reaction condition, small energy consumption, no pollution, and a high mercaptan removal rate. This method uses active olefins in a gasoline fraction to react with mercaptans having a low molecular weight to produce a thioether compound having a high boiling point, so that the low-molecular-weight mercaptan can be transferred from the light gasoline fraction to the heavy gasoline fraction so as to achieve the purpose of removal of mercaptans from the light gasoline fraction, wherein the mercaptan conversion catalyst is a key factor of the technique.
Currently, most mercaptan conversion catalysts use well-developed catalysts in other hydrotreatment processes, which have transition metals as the major active component supported on an inert carrier such as Al2O3 and SiO2 for use. U.S. Pat. No. 5,510,568, owned by Chemical Research & Licensing Ltd., discloses a process for removing mercaptans and hydrogen sulfide from cracked naphtha, in which the catalytic distillation column was loaded with supported Pd/Al2O3, which can catalyze the reaction of hydrogen sulfide and low-molecular-weight mercaptans with dienes in the naphtha to produce a heavier sulfocompound, which was further transferred into the C6+ fraction, so that a sulfur-free light naphtha component was obtained from the top of the reaction distillation column. The catalyst had a Pd load of 0.27-0.33 wt %. The starting materials for this reaction had a mercaptan content of 100,000 ppm and a diene content of 0.254 wt %, and were reacted and distilled at 265° F. in the catalyst bed. After the reaction, the light naphtha fraction as an outflow from the top of the reaction distillation column had a mercaptan content of 6,000 ppm and a reduced diene content of 0.001 wt %. However, the Pd/Al2O3 catalyst requires a high operational temperature, and may easily cause saturation and polymerization of the active unsaturated olefins in LPG if used for LPG mercaptan conversion. Moreover, the catalyst is present in a hydrogenated form during the reaction, and thus it is necessary to inject a large quantity of hydrogen gas into the reactor to keep Pd/Al2O3 in an activated state; and the reduced Pd-based catalyst is also sensitive to sulfides in the starting materials, and thus a high sulfur content may cause deactivation and poisoning of Pd/Al2O3, which affect its performance in mercaptan removal and dienes removal by hydrogenation.
Since nickel as a transition metal has strong hydrogenation activity and sulfur-poisoning resistance and lower production cost than the noble metal palladium, it has been widely applied in hydrogen desulfurization. U.S. Pat. No. 5,851,383 discloses a process for mercaptan removal by thioetherification of light hydrocarbons and diene removal by selective hydrogenation, in which a supported monometallic Ni/Al2O3 catalyst, which had an elemental Ni content of 15-35 wt % and can catalyze both thioetherification of mercaptans and olefins and selective hydrogenation of dienes, was loaded in a fixed bed reactor. The starting materials containing mercaptans and dienes were contacted with the Ni/Al2O3 in the fixed bed reactor, and operated at a temperature of 125° C. and pressure of 4.1 MPa. After the reaction, the product was further split via a rectifying column to obtain a light component having a mercaptan removal rate of 100% and a dibutene removal rate of 99%. However, this patent does not disclose a method for preparing the catalyst and the physical-chemical properties of the catalyst. In addition, the nickel-based catalyst required a high reaction temperature, which may easily cause polymerization of the active unsaturated hydrocarbons in the starting materials.
LPG contains a large quantity of highly reactive olefins which are easily saturated and polymerized at a high temperature. Thus, saturation and polymerization of these olefins can be avoided and the energy consumption can be reduced if when the operation is conducted at a low temperature. Therefore, it may be very necessary to develop a mercaptan conversion catalyst which has high activity and high stability at a low temperature.